1. Field of the Invention
The present invention relates to a photoelectric converter for converting incident light into an electric signal.
2. Description of the Related Art
An image sensor is used in an image reading apparatus such as a facsimile machine or an image scanner, and in the image sensor, a photoelectric converter is used. In the photoelectric converter, a plurality of photoelectric conversion blocks corresponding to the number of pixels are used. FIG. 5 is a circuit diagram illustrating a conventional photoelectric conversion block.
A photoelectric conversion block 50 includes a signal øR terminal (not shown), a signal øT terminal (not shown), a signal øSCH terminal (not shown), and nodes C to F. In addition, the photoelectric conversion block 50 includes a photoelectric conversion element 51, a reset switch 52, an amplifier 53, a transfer switch 54, a capacitor 55, a driver circuit 56, and a channel selection switch 57. The node E is connected to a common read line (not shown).
A signal øR becomes high, and the reset switch 52 is turned on, whereby an output voltage of the photoelectric conversion element 51 is reset to a reset voltage Vrst. A signal øT becomes high, and the transfer switch 54 is turned on, whereby the reset voltage Vrst before light receiving is amplified by the amplifier 53 and transferred to the capacitor 55 to be stored therein. After the storage, the signal øT becomes low, and thus the transfer switch 54 is turned off. A signal øSCH becomes high, and the channel selection switch 57 is turned on, whereby the output voltage (reset voltage Vrst) of the amplifier 53 before light receiving, which has been stored in the capacitor 55, is read into the common read line.
The photoelectric conversion element 51 in which the output voltage has been reset to the reset voltage Vrst lowers, when receiving incident light, the output voltage according to an amount of the received light. When the photoelectric conversion element 51 receives light for a predetermined period of time, the signal øT becomes high, and thus the transfer switch 54 is turned on. Accordingly, the output voltage after light receiving is amplified by the amplifier 53 and transferred to the capacitor 55 to be stored therein. After the storage, the signal øT becomes low, and thus the transfer switch 54 is turned off. The signal øSCH becomes high, and the channel selection switch 57 is turned on, whereby the output voltage of the amplifier 53 (output voltage of the photoelectric conversion element 51) after light receiving, which has been stored in the capacitor 55, is read into the common read line.
As described above, the photoelectric converter detects a differential voltage between the reset voltage Vrst before light receiving and the output voltage after light receiving, and reads an image (for example, see JP 2004-282716 A).
In recent years, for reading an image at high speed, for example, for reading an image at double speed, in some cases, two common read lines are prepared, and the nodes E of a plurality of the photoelectric conversion blocks 50 are alternately connected to one of the two common read lines and the other thereof, to thereby produce parallel output.
Further, in order to realize switching between image resolutions, for example, in order to perform switching between a resolution of 50% and a resolution of 100%, there are provided two modes in some cases. One is the mode in which the channel selection switches 57 of the plurality of photoelectric conversion blocks 50 are turned on one by one, and the other is the mode in which the channel selection switches 57 are turned on in twos, and voltages of the nodes E of the adjacent two photoelectric conversion blocks 50 are equalized in the same common read line.
However, in the case where high-speed image reading as described above is realized, the common read lines to which the nodes E of the adjacent two photoelectric conversion blocks 50 are connected are different from each other. For this reason, the voltages of those nodes E cannot be equalized in the same common read line, whereby the switching between image resolutions as described above cannot be realized.